#include<stdio.h>
int main()
{
    int point=0;
    scanf("%d",&point);
    if(point>=0 && point <=100)
	{
		switch(point/10)
    	{
			case 10:
        	case 9:printf("A");break;
        	case 8:printf("B");break;
       		case 7:printf("C");break;
        	case 6:printf("D");break;
        	case 5:
        	case 4:
        	case 3:
        	case 2:
        	case 1:
        	case 0:printf("E");break;
		}
	}
	else 
	{
		printf("F");
	}
    return 0;
}
	
